druntime vs phobos

Don't worry about that. You will only be using Phobos2 with D2.
druntime was split out of Phobos, to provide the bare minimum (GC, arrays,
...) of D, so it was easier to use without Phobos or with a different
library like Tango. Both are currently used and actively developed.
druntime is compiled right into Phobos2, so you will probably not even
find it as a separate file in your installation of D2, but the core.*
modules are generally part of druntime. That is where you find the garbage
collector and some basic OS bindings amongst other things.